The adverse effect of low intensity, small band UV-B irradiation (lambda =
305 +/- 5 nm, I = 300 mW m(-2)) on PS II has been studied by comparative me
asurements of laser flash-induced changes of the absorption at 325 nm, Delt
aA(325)(t), as an indicator of redox changes in Q(A), and of the relative f
luorescence quantum yield, F(t)/F-o, in PS II membrane fragments. The prope
rties of untreated control were compared with those of samples where the ox
ygen evolution rate under illumination with continuous saturating light was
inhibited by up to 95%. The following results were obtained: a) the detect
able initial amplitude (at a time resolution of 30 mus) of the 325 nm absor
ption changes, DeltaA(3)25, remained virtually invariant whereas the relaxa
tion kinetics exhibit significant changes, b) the 300 mus kinetics of Delta
A(3)25 dominating the relaxation in UV-B treated samples was largely replac
ed by a 1.3 ms kinetics after addition of MnCl2, c) the extent of the flash
induced rise of the relative fluorescence quantum yield was severely dimin
ished in UV-B treated PS II membrane fragments but the relaxation kinetics
remain virtually unaffected. Based on these results the water oxidizing com
plex (WOC) is inferred to be the primary target of UV-B impairment of PS II
while the formation of the 'stable' radical pair P680(+.)Q(A)(-.) is almos
t invariant to this UV-B treatment.